Oil field operators demand access to a great quantity of information regarding the parameters and conditions encountered downhole. A wide variety of logging tools have been and are being developed to collect information relating to such parameters as position and orientation of the bottom hole assembly, environmental conditions in the borehole, and characteristics of the borehole itself as well as the formations being penetrated by the borehole. Among such tools are acoustic logging tools, which generate acoustic signals at one end of the tool that are transmitted through the formation to receivers at the other end of the tool. By measuring the arrival times of the different types of waves at the receivers it is possible to assess the makeup of the formation between the transmitters and the receivers.
Among the waves that are propagated are pressure waves (P-waves), shear waves (S-waves), Rayleigh waves, mud waves and Stoneley waves. The Rayleigh, mud and Stoneley waves generally propagate much more slowly than P-waves and S-waves and thus arrive at the receivers after the P-wave and S-wave (generally the waves of interest). Because of this, the other three slower wave types can easily be masked. P-waves and S-waves, however, can propagate along the body of the tool, thus requiring a tool designer to implement structures in the tool to attenuate and/or slow the propagation of such waves to avoid interference with the reception of these waves through the formation.
Designing an acoustic logging tool that is robust enough for a downhole environment while still providing acoustic isolation between the transmitters and receivers of the tool can be challenging. Many materials that provide good acoustic isolation, attenuation and/or wave propagation delay are soft and not as strong as other materials that may be preferred for use in a tool housing. Further, structures used to slow down and scatter acoustic waves frequently use gaps that produce a tool surface that is not smooth, can reduce tool strength, can become clogged with debris, and can interfere with and degrade the quality of received signals of interest. And neither the materials nor the structures currently in use address the need for acoustic tools that are long enough to provide adequate transmitter and receiver separation while still being navigable through boreholes that contain sharp bends, doglegs or other abrupt deviations.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.